1. Technical Field
This invention relates to resin matrix materials having isotropically oriented fibers and methods for producing these materials, and more particularly to such materials and methods which employ so-called "fiberballs".
2. Background Art
Resin matrix materials, which are essentially composites of resin and fiber, have a variety of applications. Particular to the present invention are resin matrix materials which provide enhanced thermal conductivity, electrical conductivity, dielectric loss characteristics, and heat dissipation capacity. These materials include structural adhesives, elastomer gap fillers, sealants and the like, wherein the above-mentioned enhanced properties are desired. In the past, resin based materials requiring enhanced thermal or electrical conductivity, or dielectric loss characteristics, have been produced by mixing fibers or particulates with the desired properties into various resins. The resins are chosen to impart the desired mechanical characteristics to the finished product, such as viscosity, bonding strength or rigidity, to name a few. Typically metallic whiskers or carbon fibers or particulates have been added to the resin to create the desired conductivity or dielectric loss properties. Various fiber lengths and packing percentages have also been used to create the desired effect.
Uniform conductivity has not been obtainable by these aforementioned methods due to the random orientation of the fibers in the materials. For instance, suppose the desired effect is increased thermal conductivity. Heat is transferred through the material largely along the fibers. Fibers oriented randomly and discretely form inherent thermal resistances at fiber/matrix interfaces throughout the material. This randomness limits the usefulness of the material as an enhanced thermal conductor. Resin matrix materials with random fiber orientations have limited usefulness as an enhanced electrical conductor, as well, for the same reasons.
Some attempts have been made to orient fibers in specific directions within the resin. However, the net effect of this is to improve the conductivity in only the direction of the fibers. In fact, the conductivity in directions perpendicular to the oriented fibers is actually worsened.
Similar problems exist when using resin matrix materials having random or directional fiber alignment in dielectric loss applications. Materials with dielectric loss characteristics are typically used to dissipate an electric field applied to the material. In a sense, the fibers in the material act like individual resistors converting the electrical energy to heat. In many applications, the key to a useful resin matrix material with dielectric loss characteristics is uniformity. The loss characteristics must be the same no matter what direction the electric field is applied. However, a fiber's orientation in respect to the direction of the impinging electric field will effect its dissipative capability. Therefore, in the case of randomly orientated fibers, the overall dielectric loss characteristic of the resin matrix material will vary depending on the angle of incidence of the electric field. This is unacceptable in situations where uniformity is essential. Likewise, resin matrix materials with fibers oriented in particular directions will vary in its dielectric characteristics depending on the direction of the electric field because the fibers will not dissipate the electrical energy to same degree in one direction as compared to another.
Therefore, what is needed is a resin matrix material having enhanced conductivity or dielectric loss characteristics which is isotropic in nature. In other words, the material should exhibit the same degree of thermal and/or electrical conductivity, or dielectric loss, regardless of the where the heat or electrical energy is applied.
The formulation of heat dissipating resin matrix materials used for passive thermal management of transient heat sources has previously involved the addition of encapsulated phase change materials to the resin. Typically, these encapsulated materials are glass microballoons filled with wax. Heat energy transferred to the material is dissipated when the wax inside the microballoons melts. The rate of heat dissipation in this system is limited in part by the speed at which the heat energy is transferred from the resin to the wax. Ideally, the surface area of the interface between the resin and wax should be maximized to maximize this rate of transfer. However, in the case of wax-filled glass balloons, the available heat transfer surface area is limited to that of the glass balloon surrounding the wax. This is the reason these glass balloons must be so small. Many smaller balloons dissipate heat faster than fewer larger balloons, assuming some maximum packing factor for each, because the surface area of wax-to-glass is larger in the case of the more numerous smaller balloons. Unfortunately, the production of these smaller balloons is expensive.
Therefore, what is needed is a phase change material for incorporating into the resin of a heat dissipating resin matrix material such that the available heat transfer surface area between the resin and the phase change material is maximized and exceeds that provided by the conventional glass micro-balloons, and which is less expensive to produce.